Another plot bunny I'm putting up for adoption.
When Serenity sent her daughter and the Senshi forward in time, something went slightly wrong. Most of them ended up being born within a very narrow time frame in the same general area, so that they all wound up roughly the same age and lived in the same general area of a specific district of Tokyo. Only the princess did not end up growing up in Juuban. She was born a few years earlier, in Unimari, as a girl named Nanoha Takimachi.
At the age of 9, Nanoha began her canon career as a magical girl, searching for Jewel Seeds. One day, she decided that she had been working too much and decided to take a day off. That afternoon, a Jewel Seed activates and ends up wrecking a sizable part of town. Nanoha decides that this incident means that she can't afford to take breaks and eventually goes on to become a workaholic, training intensively whenever she is not needed to be actively on duty.
Fast forward to the time period between As and StrikerS. Nanoha's extreme work ethic catches up to her. Her body, strained continuously by the fact that she never takes any time off, collapses in the middle of a fight. Before she can recover, she is critically injured. She spends the next several months healing and relearning how to walk. Once she is released from the hospital, she is ordered to take some of the leave time she's been accumulating for the past several years. Her friend/lover Fate goes along with her, both to enjoy some vacation time and to make sure that Nanoha actually does get some rest. For a place to relax, they choose a seemingly quiet place on Nanoha's homeworld. A place that is close enough by train so Nanoha can visit family and friends, but won't be obliged to spend all her time with them. A little part of Tokyo called Juuban.
Meanwhile in Juuban, a pair of talking cats are getting worried. They've located the Inner Senshi and Prince Endymion, but there is no sign of the Moon Princess anywhere. The Senshi have been holding their own against Beryl's forces, but they can't take the offensive until they find the Princess and the Crystal. Then one day, Luna finds her. She tries to contact the girl, but is driven away by a woman with blond hair and red eyes.
No matter what the moon cats try, every attempt to reach the princess is intercepted by the woman, who is unimpressed by the sight of talking cats and seems willing to feed them to her talking dog. They watch the princess from a distance, and realize that she is in a relationship with the other woman, and shows no romantic influence in any male, let alone her destined prince. They come to a conclusion. The red-eyed woman is a witch who has ensorcelled the princess for some reason, and is trying to keep them away to ensure that they don't free her. They go to gather the Senshi.
Fate, on the other hand, knows exactly what a talking animal means. It means that they are either a familiar (Like Arf), or a shapeshifting mage (Like Yuuno). If one of those is trying to contact Nanoha, then they must be trying to inform her of something that would make Nanoha feel obliged to go back to work. Since there doesn't seem to be any problems that require TSAB intervention in the area (Youma are not a dimensional problem, and while Mettallia and the Crystal might be a potential dimensional threat and a Lost Logia respectively, Fate doesn't know about them), she sees no reason why they need to get involved with local issues when the local experts are doing a perfectly good job in keeping things under control. Fate knows that if Nanoha returns to the field before she has fully recovered, she could very well end up wrecking her health permanently, so she will do whatever it takes to protect her friend from her workaholic tendencies by keeping the cats away, even if she has to do so by letting Arf use them as chew toys. If the cats want TSAB help that badly, then they can request it through normal channels.
And so four relatively inexperienced Senshi prepare to face off against one veteran mage over the fate of a woman who is totally unaware of what is going on, both sides feeling that they have Nanoha's best interests in mind.